1. Field of the Invention
This invention relates to the use of an interface circuit to convert a rectified alternating current (AC) signal to a sinusoidal AC signal in a circuit breaker electronic trip unit (ETU).
2. Description of the Related Art
Currently, large circuit breaker electronic trip units include an iron core current transformer and an air core current transformer wherein the iron core current transformer supplies power to the ETU and the air core current transformer provides a sinusoidal signal to perform protection and metering functions. The ETU uses the air core current transformers to detect the magnitude of the current flowing through each phase of a multi-phase circuit breaker. The output of the air core current transformer is a sinusoidal signal, for example, a 60 Hz sinusoidal signal, from which power is not derived. The sinusoidal signal is integrated and input into a microprocessor based Application Specific Integrated Circuit (ASIC). The signal input into the ASIC is typically a sinusoidal signal having a maximum amplitude of .+-.1.25 Vac riding on a .+-.2.5 Vdc relative to system ground.
Smaller electronic trip unit circuit breakers, because of limited physical space, typically include only an iron core current transformer used to provide both power and sensing to the ETU but do not include an air core current transformer for providing a sinusoidal signal to an ASIC. Therefore, the small circuit breakers usually do not have the metering capabilities as provided by the large circuit breakers. It would be advantageous to provide a small circuit breaker and ETU having only an iron core current transformer to use the ASIC microprocessor as used in the larger circuit breakers having an iron core current transformer and an air core current transformer. The smaller circuit breakers typically sell at higher volumes thereby allowing the ASIC development cost to be spread out over the increased volumes. The cost of the ASIC is thereby lowered per unit as well as providing for increased functions such as a ground fault protection to the smaller circuit breakers having only an iron core current transformer.